1. Field of the Invention
The present invention relates in general to an inkjet recording apparatus and an ink cartridge, and more particularly to an inkjet recording apparatus and an ink cartridge which makes it possible to accurately detect that an amount of remaining ink is reduced to a predetermined amount.
2. Discussion of Related Art
There is known an inkjet recording apparatus, as disclosed in U.S. Pat. No. 6,702,433 (corresponding to JP-A-2002-234180), in which an amount of remaining ink is detected by a pair of electrodes. One of the pair of electrodes is provided by an ink drawing hollow needle for drawing the ink from an ink reservoir, while the other of the pair of electrodes is provided by an air introducing hollow needle for introducing an atmospheric air into the ink reservoir. The pair of electrodes provided by the respective hollow needles are disposed in a bottom portion of the ink reservoir, in substantially parallel with each other. The air is introduced into the ink reservoir through the air introducing hollow needle when the ink is drawn from the ink reservoir through the ink drawing hollow needle. An amount of the air thus introduced into the ink reservoir corresponds to an amount of a reduction in the pressure in the ink reservoir which is caused by the discharge of the ink from the ink reservoir.
In this instance, the air introduced into the ink reservoir through the air introducing hollow needle takes the form of bubbles, since the air introducing hollow needle is immersed in the ink mass, as shown in FIG. 7A. When a certain amount of the ink has been drawn from the ink reservoir through the ink drawing hollow needle 351, namely, when a top surface of the ink mass has become lower than an upper end of a tubular partition wall 364 which surrounds the air introducing hollow needle 352, an electrical continuity between the two hollow needles 351, 352 via the ink is lost, whereby a so-called “cartridge empty” is detected. However, after the detection of the cartridge empty, the two hollow needles 351, 352 are likely to be electrically connected to each other via the air bubbles overflowing the tubular partition wall 364 and deposited in the vicinity of the upper end of the tubular partition wall 364, as shown in FIG. 7B. This electrical connection between the needles 351, 352 via the overflowing air bubbles undesirably provides an erroneous detection or determination that the ink remains in the ink reservoir. Such an erroneous determination causes the air to be delivered to a print head of the inkjet recording apparatus, so that the print head having thus sucked up the air is not likely to satisfactorily perform a printing operation.
There is also known an inkjet recording apparatus in which an ink cartridge and a buffer tank provided under the ink cartridge are held in communication with each other via an ink supply passage and an air introduction passage. In this arrangement, a back pressure acting on a print head of the apparatus is held constant irrespective of an amount of the ink remaining in the ink cartridge, while an amount of the ink remaining in the buffer tank is detected.
As an example of such an inkjet recording apparatus, U.S. Pat. No. 6,702,427 (corresponding to JP-A-2002-307711) discloses an apparatus in which the ink supply passage and the air introduction passage are respectively provided by an ink drawing hollow needle and an air introducing hollow needle which are both formed of a conductive material. In this inkjet recording apparatus, the air introducing hollow needle 552 has a lower end which is given a height larger than a lower end of the ink drawing hollow needle 551, as shown in FIG. 12. When a top surface of mass of the ink left in the buffer tank 503 becomes lower than the lower end of the air introducing hollow needle 552 as a result of consumption of the ink, the air is introduced into the ink cartridge through the air introducing hollow needle 552 while the ink is supplied to the buffer tank 503 from the ink cartridge 502. Thus, the height of the top surface of the ink mass in the buffer tank 503 is held substantially constant, whereby the back pressure acting on the print head is held constant irrespective of the amount of the ink remaining in the ink cartridge 502. Meanwhile, the amount of the ink remaining in the buffer tank 503 can be detected by detecting change in an electrical resistance between the two hollow needles 551, 552.
In this inkjet recording apparatus, it is determined that the buffer tank 503 is empty or near empty, provided that a pass establishing the electrical continuity between the two hollow needles 551, 552 within the ink cartridge 502 is lost and that a pass establishing the electrical continuity within the buffer tank 503 is also lost. However, where printing operations consuming a large amount of ink are consecutively carried out while the buffer tank 503 is almost empty, a large number of bubbles W are formed by introduction of the air into the ink cartridge 502 through the hollow needle 552. The formed bubbles W are likely to stay in the vicinity of the hollow needles 551, 552, whereby the hollow needles 551, 552 are likely to be electrically connected to each other via the remaining bubbles W.
As shown in FIG. 12, the top surface of the ink mass in the ink cartridge 502 has been lowered to be lower than a partition wall 564 surrounding the air introducing hollow needle 552, and the top surface of the ink mass in the buffer tank 503 has been also lowered to be lower than the lower end of the air introducing hollow needle 552. In this instance, the ink no longer forms an electrical path establishing the continuity between the hollow needles 551, 552. However, the hollow needles 551, 552 can be electrically connected to each other via the bubbles W which have been formed as a result of the introduction of the air into the ink cartridge 502 and which overflows the partition wall 564. This state provides an erroneous determination that the ink still remains in the ink reservoir, although the buffer tank 503 is actually almost empty. That is, in the conventional apparatus, an accurate detection of the ink remaining amount has been difficult.